Jets, Not Neutrinos, May Cause Supernova Explosions, Scientists Say

Austin, Texas -- Astrophysicists at the University of Texas at Austin and
the Naval Research Laboratory (NRL) in Washington, D.C., have developed a
new theory of how supernovae explode, based on observations made at the
University of Texas at Austin McDonald Observatory. The results were
published in the Astrophysical Journal Letters on October 20 by Alexei
Khokhlov, Elaine Oran, and Almadena Chtchelkanova of NRL and Peter
Hoeflich, Lifan Wang, and J. Craig Wheeler of the University of Texas.

"Combining Texas observations with the cutting-edge numerical techniques
at NRL has pointed the way to a new idea," says Wheeler, the Samuel T. &
Fern Yanagisawa Regents Professor in Astronomy at the University of Texas
at Austin. "We think that jets cause a major class of supernova explosions."

Supernovae are caused by the explosion of a massive star, and the explosions
have been thought to arise through one of two mechanisms. In the first type,
called Type Ia, massive stars can explode like a stick of dynamite, leaving
no collapsed remnant. Astronomers use Type Ia supernovae as "standard
candles" to measure distances in the Universe, and studies of Type Ia
supernovae have suggested that the expansion of the Universe is
accelerating.

Other types of supernovae involve the collapse of the center of an
especially
massive star to form an extremely dense object, either a neutron star or,
perhaps in some circumstances, a black hole. The formation of a neutron
star is thought to be more common. These types of supernovae are called
Type Ib and Ic and Type II.

Astronomer Lifan Wang, a Hubble Postdoctoral Fellow at the University of
Texas at Austin, has studied all types of supernovae for several years,
primarily using the 2.1-meter Otto Struve Telescope at McDonald
Observatory. Wang's work has focused on determining whether the light
of supernovae is polarized that is, if the light waves given off by
supernovae are aligned in certain directions. If a supernova's light is
expanding uniformly in all directions, there is no polarization. There will
be measurable polarization if light from the parts of the supernova is
spreading asymmetrically.

All the supernovae Wang has examined that are thought to arise from
core collapse -- the Type Ib and Ic and Type II supernovae -- have been
substantially polarized, and hence substantially "out-of-round." At the
same time, all the Type Ia supernovae have shown little or no polarization.

For the polarized supernovae, Wang has identified a trend suggesting that
the closer one looks to the center of a supernova explosion, the larger the
asymmetry found. In many cases, his data suggest, the explosion must be
occurring strongly along a preferred axis. The explosion must be bipolar.
"These observations cannot be explained by current theory," says Wang,
"so a new theory was needed."

When the core collapses, a neutron star forms before any explosion can
occur. Up to now, the theory of core-collapse supernovae has been focused
on the production of neutrinos that are generated within the newly formed
neutron star. These ephemeral particles carry off more than a hundred
times the energy required to trigger the explosion of the star. The question
has been whether they carry too much and spoil the explosion, or leave
enough energy behind to cause the explosion.

To help with a new theory that explains supernova formation and takes
polarization into account, Wang and his Texas colleagues turned to Khokhlov,
Oran, and Chtchelkanova of NRL, who used computer modeling to test
scenarios that could explain the newfound polarization of these supernovae.
Their models tested the idea that collapsing supernovae begin by expelling
mass and energy from the new neutron star in a strongly directional process.

"Moving mass and energy in a single direction is the operational definition
of a jet," says Wheeler. "These are jet-induced explosions."

If the new jet theory is right, the traditional questions about neutrinos
and supernovae may be irrelevant. In their calculations, Khokhlov and his
associates found that the jet punches out of the star, but also sends shock
waves sideways, sharing some of the energy throughout the star. The result
is that the entire star is blown up by the jet and the neutrinos do not need
to play any obvious role. The ejected matter is sent out in the jet and in a
pancake containing other star material. "The result is just what we need to
explain the polarization," says Peter Hoeflich, a Research Scientist at the
University of Texas at Austin, who is an expert on the flow of radiation
from supernovae.

The numerical techniques to compute the effect of a jet on a star were
developed by Khokhlov when he was at the University of Texas at Austin
and have been refined and applied to this problem at the Naval Research
Laboratory, where he is currently a Research Scientist. The computer
code developed by Khokhlov is fully three dimensional and has an
"adaptive-mesh" capability, so that it automatically computes most
carefully just where the need is greatest. This code was used by Khokhlov
and his colleagues to compute the propagation of a jet from near the
surface of a newly formed neutron star to its eruption into space.

"The next task is to better understand the origin of the jet," says Wheeler.
"The most plausible cause is the rapid rotation of the neutron star and its
strong magnetic field. We have begun to look into how the newly formed
neutron star can channel its energy up the rotation axis by magnetic jets
or intense pulsar radiation."